FTP'd Files are Corrupt

I routinely FTP files to clients - some are small, some are large, some are text, some are binary (zip, excel). This is from a Windows 7/64 laptop, using WS_FTP Pro and/or FileZilla. Recently, some clients have complained that the files they receive are corrupt. Sure enough, when I download them from the FTP site, they are corrupt (at least the text files and the zip files, the excel files appear to be ok). I re-checked the original files on my network and they are fine. I found a suggestion from similar questions asked here that transferring via binary (instead of automatic detection) may solve the problem, and that does appear to be the case (with limited testing so far). So now I'm wondering, why not specify binary transfer for ALL files? It apparently works fine even with text files, and if it is more reliable, why even bother with automatic detection or ascii transfer? Is there any downside to using binary transfer 100% of the time? Or, conversely, is there any advantage to using ascii transfer for text files?